1. Field of the Invention
The present invention relates to a propylene polymer composition. More particularly, the present invention relates to a propylene polymer composition having excellent moldability and which is useful for producing shaped articles, for example, bumpers, having excellent lacquer-bonding property, excellent resistance to mechanical impact, and satisfactory appearance.
2. Description of the Prior Art
Generally, it is known that conventional crystalline propylene polymers exhibit excellent rigidity, satisfactory resistance to thermal deformation, and satisfactory surface hardness. However, it is also known that the conventional crystalline propylene polymers exhibit not only poor mechanical properties at low temperature but also poor resistance to mechanical impact applied thereto and poor lacquer-bonding property due to the non-polarity of the copolymers. Accordingly, it has been found that the conventional crystalline propylene polymers are unsatisfactory as material for making articles having all the properties of excellent rigidity, satisfactory resistance to thermal deformation, excellent resistance to mechanical impact, and excellent lacquer-bonding property.
In order to enhance the resistance to mechanical impact at a low temperature and to enhance the lacquer-bonding property of the crystalline propylene polymers, it was attempted to blend a crystalline ethylene-propylene block copolymer with an amorphous ethylene-propylene copolymer alone or a mixture of the amorphous ethylene-propylene copolymer with various types of polyethylene.
However, in this attempt, it was found that the above-mentioned modes of blends caused the resultant copolymer blends to exhibit lower rigidity and resistance to thermal deformation than those of the crystalline ethylene-propylene block copolymers.
In order to improve the conventional propylene polymer compositions, Japanese Unexamined Pat. Publication (Kokai) No. 53-64257 (1978) discloses a resinous composition having a high impact resistance. The publication describes the resinous composition comprising a blend of a crystalline ethylene-propylene block copolymer with an amorphous ethylene-alpha-olefin copolymer and talc and being capable of providing a shaped article having significantly enhanced impact resistance at low temperature, rigidity, and lacquer-bonding property in comparison with conventional crystalline ethylene-propylene block copolymers.
The above-mentioned publication also describes various examples of polymeric compositions comprising, as a polymeric component, a crystalline ethylene-propylene block copolymer having a melt flow index of 1.5, ethylene-propylene-non-conjugated diene copolymer having a melt flow index of 0.4, or amorphous ethylene-propylene copolymer having a melt flow index of 0.8. All of the above-mentioned polymers have a very high molecular weight. Accordingly, the polymeric composition of the above-mentioned publication is very useful for producing therefrom shaped articles having excellent rigidity, resistance to thermal deformation, and impact resistance. However, the polymeric composition is disadvantageous in that the resultant shaped articles tend to have undesirable flow marks formed thereon, resulting in an unsatisfactory appearance, and exhibit an unsatisfactory lacquer-bonding property. Therefore, the resultant shaped articles can only be used in special fields in which the appearance of the articles is not important and are not suitable for use as parts, particularly, bumpers, of cars, the satisfactory appearance of which is just as important as the mechanical properties thereof.
That is, the prior art failed to obtain shaped articles having not only satisfactory rigidity and impact resistance but also superior lacquer-bonding property and appearance by molding conventional crystalline ethylene-propylene block copolymers.